vrend/egl: allow EGL fencing for OpenGL contexts

I cannot find any limitation in EGL_KHR_fence_sync or
EGL_ANDROID_native_fence_sync that limits our abilty in vrend to
support EGL fencing for OpenGL contexts (in addition to GLES contexts,
which vrend already supports).

EGL_ANDROID_native_fence_sync is critical for providing
virgl_renderer_export_fence()-like functionality, which is interesting
for obtaining a sync_file fd exported from a rendering context to give
to the host compositor (one half of explicit fencing for WSI).

Signed-off-by: Ryan Neph <ryanneph@google.com>
Part-of: <https://gitlab.freedesktop.org/virgl/virglrenderer/-/merge_requests/1168>
2 files changed